Production Leveling

Production leveling, also known as production smoothing or – by its Japanese original term – heijunka (平準化?), is a technique for reducing the muda (waste). It was vital to the development of production efficiency in the Toyota Production System and lean manufacturing. The goal is to produce intermediate goods at a constant rate so that further processing may also be carried out at a constant and predictable rate.

On a production line, as in any process, fluctuations in performance increase waste. This is because equipment, workers, inventory and all other elements required for production must always be prepared for peak production. This is a cost of flexibility. If a later process varies its withdrawal of parts in terms of timing and quality, the range of these fluctuations will increase as they move up the line towards the earlier processes. This is known as demand amplification.

Where demand is constant, production leveling is easy, but where customer demand fluctuates, two approaches have been adopted: 1) demand leveling and 2) production leveling through flexible production.

To prevent fluctuations in production, even in outside affiliates, it is important to minimize fluctuation in the final assembly line. Toyota's final assembly line never assembles the same automobile model in a batch. Instead, they level production by assembling a mix of models in each batch and the batches are made as small as possible. This is in contrast to traditional mass production, where long changeover times meant that it was more economical to punch out as many parts in each batch as possible. When the final assembly batches are small, then earlier process batches, such as the press operations, must also be small and changeover times must be short. In the Toyota Production System die changes (changeovers) are made quickly (SMED). In the 1940s changeovers took two to three hours, in the 1950s they dropped from one hour to 15 minutes, now they take three minutes.